When Synapse is terminated while running the background update to create
the `receipts_graph` or `receipts_linearized` indexes, the indexes may
be successfully created (or marked as invalid on postgres) while the
background update remains unfinished. When Synapse next starts up, the
background update will fail because the index already exists, or exists
but is invalid on postgres.
Use the existing code to create indices in background updates, since it
handles these edge cases.
Signed-off-by: Sean Quah <seanq@matrix.org>
